IANAL but my understanding is the contains your wishes about the distribution of your assets. However, the process of actually operating the will (ie probate) can take a while. Hence, you are better off moving your assets into a trust so that in the event of your passing, the trustee gets to distribute your assets according to your wishes immediately.
It does seem complicated. User sowbug has commented above that in California the trust can actually get you out of significant taxation. I would even call it unreasonable taxation (why would you tax the assets, instead of assets - liabilities, that doesn't make any sense).
